Completely meaningless phrase which has gained inexplicable global popularity. It's an incomplete thought: Back in what day? It's a redundant way of saying "Once, I..." or "I used to..." and adds words without adding any extra meaning. Similar to the equally pointless "at the end of the day..." popularised by English soccer stars.

Reminiscing, remembering fun times, remembering a time when life was more simple

We'd play hide-n-seek only after dark, back in the day.
Back in the day, I could beat Mario Brothers in 10 minutes.
Back in the day, I'd bug my sister 'til she ran to her bedroom bawling, then laugh hysterically.